1. Field of the Invention
This invention relates to a liquid crystalline composition and also to a TN type liquid crystal display element, and more particularly it relates to a liquid crystalline composition having an improved threshold voltage, temperature dependency (hereinafter often abbreviated to Vth) of display elements.
2. Description of the Related Art
In recent years, as the performance of liquid crystal display devices has improved, their fields of applications have also been extended. Liquid crystal displays during their earlier stages of development had been used mainly for handy type electric calculators, watches, clocks, etc. due to the specific feature of low power consumption of the displays. Recently, however, as liquid crystal materials have been developed, applications of liquid crystal display devices have come to be rapidly expanded in various fields such as those of outdoor display devices or instrumental panels for automobiles having broad service temperature ranges or those of large scale displays such as hand-held computors, computor terminal equipment, etc.
When this is viewed from the driving mode of display elements, the mode has been transferred from the static driving mode of watches, etc. at the earlier stages to the multiplex driving mode of 1/2 to 1/4 duty, etc. as in the case of electric calculators, etc. by which the capacity of information display is increased. At present, the driving mode has come to be transferred from 1/16 duty to 1/64 duty or a higher multiplex driving mode by which word processors as well as graphic displays are effected.
In the case of such a high multiplex driving mode, an operational restriction which does not occur at all in the case of the static driving mode is present in principle. Namely, in the voltage-levelling method generally employed in the multiplex driving mode, crosstalk at a half-selected or unselected element is liable to occur; and moreover, the higher the multiplexing of the driving mode becomes, the less the allowance of the operation voltage i.e. the operation margin becomes and the more liable the crosstalk is to occur.
The following three factors affect the operation margin in a highly multiplexed display:
(1) temperature dependency of threshold voltage, PA1 (2) viewing angle dependency of threshold voltage and PA1 (3) steepness of voltage-brightness characteristic. PA1 (1) A temperature at which the twist sense is reversed is inevitably present. In general, there is a substance which is liable to cause a reverse twist on the lower temperature side, while forming 270.degree. twist on the higher temperature side. PA1 (2) Further, since right-twisted and left-twisted, optically active substances whose twist senses are offset to each other are mixed together, addition thereof in small quantities elongates the intrinsic pitch; hence in order to obtain a desired intrinsic pitch P, increase in the quantities of the optically active substances added is by all means compelled. Thus, the nematic phase temperature range and viscosity of the original nematic liquid crystal, and the specific features of the resulting liquid crystal display element such as threshold voltage Vth, etc. are considerably varied. In particular, this has a bad influence upon the steepness of voltage-brightness characteristic, and since the viscosity also generally rises due to increase in the amount of chiral dopants, there is a tendency that the response time of the liquid crystal cell is prolonged. PA1 (3) When expensive optically active substances as compared with usual nematic liquid crystals are added in large quantities, the ultimate liquid crystal composition also becomes considerably expensive.
Among these factors, factor (2) depends on the product of the optical anisotropy value (.DELTA.n) of liquid crystal materials by the cell gap (d) of display elements (the value of .DELTA.n.times.d), and factor (3) also depends greatly on the cell constitution of a TN type LCD; hence it is difficult to effect a notable difference by way of the materials.
Whereas as to the factor (1), the influence of the materials thereon is greatest since the operation temperature range of display elements is also related thereto. In order to improve factor (1), such a countermeasure directed to liquid crystal materials themselves is mentioned that by increasing the proportion of a liquid crystal component having a negative dielectric anisotropy value (hereinafter often abbreviated to .DELTA..epsilon.), the temperature dependency of the threshold voltage is reduced (see Japanese patent application laid-open No. Sho 54-83694/1979). However, even when such liquid crystal materials are used, reduction in the dependency has not been to such an extent that the voltage margin is effected in a narrow range of 0.degree. to 40.degree. C. under 1/2 to 1/4 duty so that it has been difficult to secure the voltage margin over a broad temperature range. In other words, there is a limitation to such a method of increasing the content of liquid crystal components having negative .DELTA..epsilon. to thereby reduce the temperature dependency of the threshold voltage due to liquid crystal materials; thus an effectiveness with a leap cannot be expected.
In the case of TN type display elements, it has been quite usually carried out to add a slight quantity of an optically active substance called a chiral dopant thereto and thereby determine the twist sense of liquid crystal molecules within a cell to suppress the reverse twist of the liquid crystal molecules and thus keep constant the display grade of the TN type liquid crystal display elements. When an optically active substance is added as above, the resulting nematic liquid crystals form a helical structure. The driving threshold voltage of TN type display elements is influenced by the helical intrinsic pitch of the liquid crystal materials used for the elements when the cell thickness and the temperature are constant.
Japanese patent application laid-open No. Sho 55-38869/1980 discloses such a liquid crystal mixture that when a right-twisted helical, optically active substance and a left-twisted helical, optically active substance are added in certain proportions to a nematic liquid crystal, reduction in the threshold voltage on the higher temperature side is prevented and also the temperature dependency thereof is reduced. In the above reference, a liquid crystal material is diclosed which makes the temperature dependency of the threshold voltage zero in the range of 0.degree. to 40.degree. C. However, such a mode using chiral dopants consisting of two components having opposite helical twist senses has the following problems:
Accordingly, such a mode using two chiral dopants for opposite twist senses has not been employed so much. Thus, for practical use, one is compelled to employ a means of imparting a temperature-compensating function to the driving circuit for LCD to thereby control the voltage impressed to the liquid crystal cell.
As described above, while it has been earnestly desired to reduce the temperature dependency of the threshold voltage originated from nematic materials themselves, conventional materials which can reduce the temperature depenceny of Vth over a broad temperature range have been scarcely present.